A blanket of Azolla fern coats a large section of Lake Anza in Tilden Regional Park in Berkeley.
Lake Anza covered in Azolla fern on Aug. 24, 2023. Credit: Iris Kwok

When the water’s clear for swimming at Lake Anza, you shouldn’t delay splashing in. 

Less than four months after the Tilden watering hole opened to swimmers for the first time since 2019, it’s been closed once again.  

Before this spring, the beloved swimming spot had been closed for four years due to several factors, including ADA improvements, construction, harmful algae blooms and an “explosive” growth of the nontoxic reddish green Azolla fern last year.

The fern receded this past winter, likely due to the heavy winter rainstorms, and the East Bay Regional Park District reopened Lake Anza for weekend swimming on April 29.

But the squishy fern, which tends to grow when temperatures are warm, never completely went away, and earlier this month it crept into the area roped off for swimming — prompting the park district to close the lake on Aug. 17. (The lake’s swim season usually ends in mid-September and remains closed throughout the fall and winter due to safety concerns.) 

Close-up views of the Azolla fern coating Lake Anza. Credit: Supriya Yelimeli

Despite its unsightly appearance, the mat of floating vegetation is believed to be harmless while alive, and may help reduce the presence of toxic algae by outcompeting the toxic cyanobacteria, or blue-green algae, for nutrients and cooling down the lake’s temperature, since cyanobacteria thrive in warm and stagnant water. However, there is a concern that as the Azolla fern decays, it could add nutrients to the water that could contribute to a toxic bloom.

The cyanobacteria is still present in parts of the lake, but is not currently toxic, Becky Tuden, the park district’s environmental services manager, wrote in an email Thursday. (Scientists don’t know what causes the cyanobacteria to at times become toxic.)

In June, EBRPD approved the purchase of a $250,000 aquatic harvester machine, which will be used to remove excessive aquatic weed vegetation in lakes across the district and make room for recreation.

It’s unclear whether the lake will reopen for swimming again this year.

“I don’t have a detailed timeline for when the lake will be harvested and how long it is anticipated to take,” Tuden wrote. “My understanding is that when the public safety department is certain the azolla no longer poses as safety risk they will reopen the lake per the swim season.”

The surface of Wildcat Creek has streaks of light green on it.
Streaks of cyanobacteria, or blue-green algae, at the mouth of Wildcat Creek, which flows into Lake Anza. Credit: Iris Kwok

Iris Kwok covers the environment for Berkeleyside through a partnership with Report for America. A former music journalist, her work has appeared in the San Francisco Chronicle, KQED, San Francisco Examiner...